Acoustic touch position sensors are well known. A common system includes two sets of transducers, each set having a different axis aligned respectively with the axes of a physical Cartesian coordinate system defined by a substrate. An acoustic pulse is generated by one transducer, propagating as a Rayleigh wave along an axis which intersects an array of reflective elements, each element angled at 45.degree. and spaced corresponding to an integral number of wavelengths of the acoustic wave pulse. Each reflective element reflects a portion of the wave along a path perpendicular to the axis, across an active region of the substrate, to an opposing array and transducer which is a mirror image of the first array and transducer. The transducer in the mirror image array receives an acoustic wave consisting of superposed portions of the wave reflected by the reflective elements of both arrays, directed antiparallel to the emitted pulse. Wavepaths in the active region of the sensor have characteristic time delays, and therefore a wavepath or wavepaths attenuated by an object touching the active region may be identified by determining a timing of an attenuation in the composite returning waveform. A second set of arrays and transducers are provided at right angles to the first, and operate similarly. Since the axis of a transducer corresponds to a physical coordinate axis of the substrate, the timing of an attenuation in the returning wave is indicative of a Cartesian coordinate of a position on the substrate, and the coordinates are determined sequentially to determine the two dimensional Cartesian coordinate position of the attenuating object.
The applicability of such systems as commonly employed is restricted by the following major limitations. First, acoustically absorptive contamination in localized regions, e.g. a water drop on a known Rayleigh-wave sensor, result in large areas of shadowing in which two-dimensional touch positions cannot be reconstructed. Second, the configurational requirements of these sensors limits their versatility with regard to shape and size. Third, reconstruction of touch coordinates may lead to ambiguities when more than one touch is applied simultaneously. Finally, such sensors provide limited touch characteristic information from which to differentiate valid touches from false touches, e.g. fingers from water drops. The present invention addresses these problems.
Present commercial touch screen products generally serve applications in which the touchscreen is an input device that is intended to be used by one user at a time. An automatic-teller-machine (ATM) banking application is typical. While many customers may sequentially use a touchscreen based automatic teller machine, each user in turn has a private dialog with the system. In contrast, few if any touchscreen products are presently available for applications in which the touchscreen is an input device that is intended to be used by more than one user simultaneously.
a. Parallel Transducer Arrays
Acoustic touch position sensors are known to include a touch panel or plate having an array of transmitters positioned along a first edge of a substrate for simultaneously generating parallel surface acoustic waves that directionally propagate through the panel to a corresponding array of detectors positioned opposite the first array on a second edge of the substrate. Another pair of transducer arrays is provided at right angles to the first set. Touching the panel at a point causes an attenuation of the waves passing through the point of touch, thus allowing interpretation of an output from the two sets of transducer arrays to indicate the coordinates of the touch. This type of acoustic touch position sensor is shown in U.S. Pat. No. 3,673,327 and WO 94/02911, Toda, incorporated herein by reference. By employing a direct acoustic path from a transmitting transducer to a corresponding receiving transducer, an acoustic path length which is approximately equal to the height or width of the substrate is provided, as shown in FIG. 1. Because the acoustic wave diverges, a portion of a wave emitted from one transmitting transducer will be incident on a set of receiving transducers, as shown in FIG. 2.
b. Reflective Arrays
In order to reduce the number of transducers required for an acoustic touchscreen, Adler, Re. 33,151, and 4,700,176, provide a reflective array for reflecting portions of an acoustic wave along incrementally varying paths. Therefore, if two such arrays are disposed opposite one another, as shown in FIG. 4, a single transmit and receive transducer will allow touch sensing along one axis of the substrate, with a maximum acoustic path length of twice the height plus width or twice the width plus height of the touch sensitive area. The maximum acoustic path length is a useful metric for acoustic touch sensors because most materials, e.g., glass, have a relatively constant acoustic power loss expressed in dB per unit length; the greater the path length, the greater the attenuation. In many cases, it is this attenuation of the acoustic signal which limits the design of the touchscreen, and therefore it is generally desired to have high acoustic efficiency in each of the touchscreen components to allow design leeway. Thus, for example, greater numbers of transducers may be selectively deployed to allow larger substrates, and likewise, with limited size substrates, acoustic paths may be folded to reduce a required number of transducers.
In order to provide a set of surface acoustic waves which propagate across a broad region of the substrate in parallel, an acoustically reflective grating having elements set at 45.degree. to the axis of the beam is disposed along its path, each element reflecting portions of the wave at right angles to the axis of propagation. The acoustic waves are then collected, while maintaining the time dispersion information which characterizes the axial position from which an attenuated wave originated. The position of a touch in the active area is thus determined by, e.g., providing another reflective grating opposite the first, which directs the surface acoustic waves as a superposed wave to another transducer along an antiparallel path, recording the time of arrival and amplitude of a wave pattern, an attenuation of which corresponds to a touch and a characteristic time corresponding to a position along the axis of the arrays. The touch, in this case, may include a finger or stylus pressing against the surface directly or indirectly through a cover sheet. See, e.g., U.S. Pat. No. 5,451,723. In addition, if the emitted wave diverges, one of the reflective arrays may be eliminated, as shown in FIG. 3, although a rectangular coordinate system is not provided. In the case shown in FIG. 3, the maximum path length is approximately the height plus the width. Acoustic touch position sensors are also known wherein a single transducer per axis is provided for emitting a surface acoustic wave, as shown in FIG. 5. In this case, the maximum path length is two times the sum of the height plus width.
The known reflective arrays are generally formed of a glass frit which is silk-screened onto a soda-lime glass sheet formed by a float process, and cured in an oven to form a chevron pattern of raised glass interruptions. These interruptions typically have heights or depths of order 1% of the acoustic wavelength, and therefore only partially reflect the acoustic energy.
Thus, with waves having surface energy, the reflecting arrays may be formed on the surface, and where wave energy is present on both sides of the substrate, these reflecting arrays may be formed on one or both sides of the substrate. Because the touch sensor is generally placed in front of a display device, and because the reflective array is generally optically visible, the reflective arrays are generally placed at the periphery of the substrate, outside of the active sensing area, and are hidden and protected under a bezel. The reflective elements of the reflective array each generally reflect of order 1% of the surface acoustic wave power, dissipating a small amount and allowing the remainder to pass along the axis of the array. Thus, array elements closer to the transmitting transducer will be subject to greater incident acoustic energy and will therefore reflect a greater amount of acoustic power. In order to provide equalized acoustic power at the receiving transducer, the spacing of the reflective elements may be decreased with increasing distance from the transmitting transducer, or the acoustic reflectivity of the reflective elements may be altered, allowing increased reflectivity with increasing distance from the transmitting transducer.
Adler, U.S. Re. 33,151, relates to a touch-sensitive system for determining a position of a touch along an axis on a surface. A surface acoustic wave generator is coupled to a sheet-like substrate to generate a burst of waves, which are deflected into an active region of the system by an array of wave redirecting gratings. According to a disclosed example, surface acoustic waves traversing the active region are, in turn, redirected along an axis by gratings to a receiving transducer. A location of touch is determined by analyzing a selective attenuation of the received waveform in the time domain, each characteristic delay corresponding to a locus on the surface. The redirecting gratings are oriented at 45.degree. to the axis of propagation, and spaced at integral multiples of the surface acoustic wave wavelength, with dropped elements to produce an approximately constant surface acoustic wave power density over the active area. The spacing between grates decreases with increasing distance along the axis of propagation from the transducer, with a minimum spacing of at least one wavelength of the transmitted wave. U.S. Pat. No. 5,329,070, U.S. Pat. No. 5,260,521, U.S. Pat. No. 5,234,148, U.S. Pat. No. 5,177,327, U.S. Pat. No. 5,162,618 and U.S. Pat. No. 5,072,427 propose specific examples of types of surface acoustic waves that may be used in the acoustic sensor system taught in the Adler patents.
Where a separate reflective array is provided to redirect acoustic waves toward the receiving transducer, these are also provided with an increasing acoustic reflectivity with increasing distance from the receiving transducer. This is to reduce signal loss with propagation of the signal toward the receiving transducer along the axis of the reflective array. Typically, array pairs are designed as mirror images of one another.
U.S. Pat. No. 4,642,423, to Adler, incorporated herein by reference, addresses pseudo-planarization techniques for rectangular touchscreen surfaces formed by small solid angle sections of a sphere. According to Adler, reflective elements are angled to excite waves along sections of great circles of the spherical surface which extrapolate to a common intersection point. This patent addresses the need for touchscreens that match the curvature of CRT faceplates, for which the radius of curvature is always large compared to the diagonal dimension of the faceplate. This patent teaches means to minimize the inherent differences between spherical geometry of a small portion of a sphere and the Cartesian plane, allowing use in conjunction with controllers that are designed for flat sensor geometry. The acoustic waves generated by the system of Adler are substantially orthogonal. Known embodiments of the Adler technology include 19 inch diagonal CRTs with a radius of curvature of 32 inches and 13 or 14 inch diagonal CRTs with a radius of curvature of 22.6 inches.
c. Two Dimensional Position Sensing
In order to receive information determinative of the coordinates of a touch, two acoustic waves, each propagating across the active region of the substrate along perpendicular axes are provided. Thus, the two axes are typically used in conjunction to recognize a valid touch, but may also be analyzed separately and non-interactively to sequentially determine a position along each of the two orthogonal coordinate axes. In these known systems, the coordinate axes of interest to the application are defined by the physical configuration of the sensor. Thus, sensor design is constrained by the requirements of the application's coordinate system.
In known systems, the system operates on the principle that a touch on the surface attenuates surface acoustic waves having a power density at the surface. An attenuation of a wave traveling across the substrate causes a corresponding attenuation of waves impinging on the receive transducer at a characteristic time period. Thus, the controller need only detect the temporal characteristics of an attenuation to determine the axial coordinate position. Measurements are taken along two axes sequentially in order to determine a Cartesian coordinate position.
Other known systems, described in more detail below, employ a single reflective array for separating as a plurality of wave paths, and superposing as a composite waveform, the signal from the transducer, through the active region, along a plurality of paths and then back to the transducer, by providing an acoustically reflective edge spaced parallel to the reflective array, causing the dispersed wave to traverse the active region twice, as shown in FIG. 5. See, U.S. Pat. No. 5,177,327, FIG. 10 and accompanying text, incorporated herein by reference.
FIG. 11 of U.S. Pat. No. 4,700,176 teaches the use of a single transducer for both transmitting the wave and receiving the sensing wave, with a single reflective array employed to disperse and recombine the wave. Such systems therefore employ a reflective structure opposite the reflective array. As a result, an acoustic wave passes through the active region twice, with consequent increased wave absorption by the touch but also increased overall signal attenuation due to the reflection and additional pass through the active region of the substrate. Thus, the acoustic wave may be reflected off an edge of the substrate or an array of 180.degree. reflectors parallel to the axis of the transmission reflective grating and reflected back through the substrate to the reflective array and retrace its path back to the transducer. The transducer, in this case, is time division multiplexed to act as transmitter and receiver, respectively, at appropriate time periods. A second transducer, reflective array and reflective edge are provided for an axis at right angles to allow determination of a coordinate of touch along perpendicular axes.
A known system by Electro-Plasma (Milbury Ohio) employs a bisected reflecting array in order to reduce an acoustic wavepath, as shown in FIG. 6A. Therefore, a maximum path length of an acoustic wave along the composite reflecting array from a transducer is about one half of the total width, with transducers each sending acoustic waves toward the bisection point. Thus, the orthogonal set of paths will be longer, with a maximum total path length of two times the height plus the width. In this system, transmitting transducers are excited individually and produce identical types of waves, portions of which travel along parallel paths, with a small overlap of acoustic wave coverage of the touchscreen in order to avoid a dead zone in the touch region. The acoustic waves follow the traditional paths corresponding to axes parallel to the Cartesian coordinate axes. A similar type system would bisect both sets of reflective arrays, as shown in FIG. 6B.
The "triple transit" system, shown in FIG. 8, provides for a single transducer which produces a sensing wave for detecting touch on two orthogonal axes, which both produces and receives the wave from both axes. In this case, the area in which touch is to be sensed is generally oblong, such that the longest characteristic delay along one path is shorter than the shortest characteristic delay along the second path, thereby allowing differentiation between the two axes based on time of reception. See, U.S. Pat. Nos. 5,072,427, 5,162,618, and 5,177,327, incorporated herein by reference. The maximum path length of the triple transit design is four times the width plus two times the height. Due to the significant difference in path lengths, the X and Y signals are non-overlapping, as shown in FIG. 9C.
d. Controller Algorithms
The wave pattern of one type of known acoustic touch sensors is dispersed along the axis of the transmitting reflective array, traverses the substrate and is recombined, e.g., by another reflective grating, into an axially propagating wave, dispersed in time according to the path taken across the substrate, and is directed to a receiving transducer in a direction antiparallel to the transmitted wave, which receives the wave and converts it into an electrical signal for processing based on signal amplitude received as a function of time. Thus, according to this system, only two transducers per axis are required. Because of the antiparaliel path, the time delay of a perturbation of the electrical signal corresponds to a distance traveled by the wave, which in turn is related to the axial distance from the transducer along the reflecting arrays traveled by the wave before entering the active area of the substrate, i.e., approximately two times the distance along the axis of the array plus the spacing between the arrays. A typical set of return waveforms is shown in FIG. 9.
The location of a touch is determined by detecting an attenuation of the received signal amplitude either in absolute terms or as compared to a standard or reference received waveform. Thus, for each axis, a distance may be determined, and with two orthogonal axes, a unique coordinate for the attenuation determined. Acoustic touch position sensors of this type are shown in U.S. Pat. Nos. 4,642,423, 4,644,100, 4,645,870, 4,700,176, 4,746,914 and 4,791,416, incorporated herein by reference.
U.S. Pat. Nos. Re. 33,151, and 4,700,176 also disclose a touch sensor system having a set of diverging acoustic paths which are incident on a reflective array having elements located along an arc and spaced to meet coherency criteria. See, Re. 33,151, and 4,700,176, FIG. 16 and accompanying text, incorporated herein by reference. This touch sensor produces a unidimensional output which corresponds to an angular position of a touch.
According to known systems, a number of algorithms are employed to determine the coordinate position of a touch. The simplest algorithm is a threshold detection, in which an amplitude of a received signal is compared to a set value. Any dip below that value is considered indicative of a touch. More sophisticated is an adaptive threshold, in which the threshold varies based on actual sets of received data, thus allowing increased sensitivity and rejection of artifacts of limited amplitude.
A control circuit may operate in a number of modes, e.g., number of transducers and configuration. In known systems having a rectangular substrate without redundancy, the number of transducers varies: 1 (triple transit); 2 (ExZec/Carroll Touch); 4 (Adler); and 6 (ElectroPlasma). There is a natural 8 transducer arrangement, not present in prior art designs, which is an extension of 6 transducer scheme in which 4 transducers are used for both X and Y axis measurements; see FIG. 6B.
Known systems also include an adaptive baseline, in which an amplitude of the normal received signal over time is stored, and the received signal is compared to a baseline having a characteristic timeframe. In this system, an artifact in one position does not necessarily reduce sensitivity at another.
Brenner et al., U.S. Pat. No. 4,644,100 relates to a touch sensitive system employing surface acoustic waves, responsive to both the location and magnitude of a perturbation of the surface acoustic waves. The system according to U.S. Pat. No. 4,644,100 is similar in execution to the system according to U.S. Re. 33,151, while determining an amplitude of a received wave and comparing it to a stored reference profile.
In order to reduce the number of transducers, the known "triple transit" system reflects the acoustic signal so that a wave emitted by a single transducer is dispersed as parallel waves along a first axis, then reflected at a right angle and dispersed as parallel waves along a second axis. These waves are then reflected back to the arrays and then back to the transducer, so that all the waves traveling along the first axis are received by the transducer prior to any waves traveling along the second axis, generally requiring an oblong substrate. The controller therefore sets two non-overlapping time windows for the received signal, a first window for the first axis and a second window for the second axis. Therefore, each time window is analyzed conventionally, and the pair of Cartesian coordinates is resolved.
A system for sensing a force of a stylus against an acoustic touch-sensitive substrate is disclosed in U.S. Pat. No. 5,451,723, incorporated herein by reference. This system converts the point-contact of the rigid stylus portion into an area contact of an acoustically absorptive elastomer, placed between the stylus and the substrate.
e. Wave Modes
"Surface acoustic waves" ("SAW"), as used herein refers to acoustic waves for which a touch on the surface leads to a measurable attenuation of acoustic energy. Several examples of surface acoustic waves are known.
The vast majority of present commercial products are based on Rayleigh waves. Rayleigh waves maintain a useful power density at the touch surface due to the fact that they are bound to the touch surface. Mathematically, Rayleigh waves exist only in semi-infinite media. In practice it is sufficient for the substrate to be 3 or 4 wavelengths in thickness. In this case one has quasi-Rayleigh waves that are practical equivalents to Rayleigh waves. In this context, it is understood that Rayleigh waves exist only in theory and therefore a reference thereto indicates a quasi-Rayleigh wave.
Like Rayleigh waves, Love waves are "surface-bound waves". Particle motion is vertical and longitudinal for Rayleigh waves. Both shear and pressure/tension stresses are associated with Rayleigh waves. In contrast, particle motion is horizontal, i.e. parallel to touch surface, for Love waves. Only shear stress is associated with a Love wave. Other surface-bound waves are known.
Another class of surface acoustic waves of possible interest in connection with acoustic touchscreens are plate waves. Unlike surface-bound waves, plate waves require the confining effects of both the top and bottom surfaces of the substrate to maintain a useful power density at the touch surface. Examples of plate waves include symmetric and anti-symmetric Lamb waves, zeroth order horizontally polarized shear (ZOHPS) waves, and higher order horizontally polarized shear (HOHPS) waves.
The choice of acoustic mode affects touch sensitivity, the relative touch sensitivity between water drops and finger touches, as well as a number of sensor design details. However, the basic principles of acoustic touchscreen operation are largely independent of the choice of acoustic mode.
f. Optimization for Environmental Conditions
The exposed surface of a touchscreen is ordinarily glass. While certain systems may include such additions, electrically conductive coatings or cover sheets are not necessary. Therefore, acoustic touchscreens are particularly attractive for applications which depend on public access to a durable touch interface.
Semi-outdoor applications, e.g., ATMs, ticket booths, etc., are of particular interest. Typically in such applications, the touchscreen is protected from direct environmental precipitation contact by a booth or overhang. However, indirect water contact, due to user transfer or condensation is possible. Thus, users coming out of the rain or snow with wet clothes, gloves or umbrellas are likely to leave occasional drops of water on the touchscreen surface. Water droplets have a high absorption of Rayleigh waves in known systems; thus, a drop of water in the active region will shadow the acoustic paths intersecting that drop, preventing normal detection of a touch along those axes.
One approach to limit water contact with the touchscreen surface is to employ a cover sheet. See U.S. Pat. No. 5,451,723. However, a cover sheet generally reduces the optical quality of the displayed image seen through the resulting sensor and leads to a less durable exposed surface. Another approach to reducing the effects of water droplets is to employ a wave mode which is less affected by the droplets, such as a low frequency Rayleigh wave, see U.S. Pat. No. 5,334,805, a Lamb wave, see U.S. Pat. Nos. 5,072,427 and 5,162,618, or a zero order horizontally polarized shear wave, see U.S. Pat. No. 5,260,521. These waves, however, also have reduced sensitivity, resulting in either reduced touch sensitivity of the touch system, increased susceptibility to electromagnetic interference, or more expensive controller circuitry.
In the case of Rayleigh waves, a lower frequency operation requires a thicker substrate, e.g., 3 to 4 wavelengths of the wave, and wider reflective arrays and transducers. The increased bulk of a sensor designed for low-frequency Raylecigh waves is typically a serious mechanical design problem. In the case of Lamb waves, a thin substrate is required, e.g., about 1 mm at about 5 MHz. These thin substrates are fragile, and Lamb waves have energy on both top and bottom surfaces, making optical bonding problematic due to signal damping. In the case of a ZOHPS wave, in contrast to a Rayleigh wave, the relative sensitivity is greater to a finger than to water droplets. Further, ZOHPS waves support limited options for optical bonding, such as RTVs (silicone rubbers) which do not support shear radiation damping.
Shear sensors have two disadvantages in cold climates. In particularly cold climates, it is important for touchscreens to sense touches of fingers of gloved hands. Shear waves have reduced sensitivity compared to Rayleigh waves thus making detection of gloved fingers more difficult. Secondly, in such climates, drops of water may freeze to form solid ice. While liquid water does not strongly couple to horizontally polarized shear waves, ice does. Thus drops of water which freeze on the touchscreen surface will cause shadowing or blinding.
There remains a need for a touch position sensor which operates reliably in the increasingly rugged environments to which such devices are deployed. There thus exists a need to supplement existing technologies in order to extend the applicability of acoustic touch sensor systems.
g. Size Constraints.
Acoustic sensors of the Adler type have been considered for use in electronic white boards; see FIG. 10 and associated text in E.P. Application 94119257.7, Seiko Epson. At present, no commercial electronic whiteboard products are available based on acoustic sensor technology. In part, this is because of size limitations for known acoustic technology.